The present invention relates to a release material, a release material article using the same, and a process for producing the release material article.
Pressure-sensitive adhesive tapes and pressure-sensitive adhesive sheets are usually obtained by providing a pressure-sensitive adhesive on the surface of a substrate. In the case of tapes, the article is typically stored in a roll form. On the back surface of the substrate, a release material is usually coated so as to protect the adhesive surface and facilitate unwinding upon use. In some cases, another substrate having a release material coated thereon is used to cover and, thereby, protect the adhesive surface.
In general, release materials are roughly classified into those materials containing silicone (xe2x80x9csilicone-type release materialxe2x80x9d) and those materials not containing silicone (xe2x80x9cnon-silicone type release materialxe2x80x9d). Silicone-type release material can provide a release strength over a wide range, from relatively low (e.g., 0.1 N/25 mm or less) to relatively high (e.g., 10 N/25 mm or more). However, silicone-type release material may readily transfer to an adhesive surface due to insufficient anchoring to the substrate, for example, a plastic substrate. The transferred silicone-type release material may give rise to contamination. For examples, in the electronics industry, this may be undesirable.
Non-silicone type release material is further classified into those materials containing fluorine (xe2x80x9cfluorine-type release materialxe2x80x9d) and those materials containing a long chain alkyl group (xe2x80x9clong chain alkyl group-type release materialxe2x80x9d). Fluorine-type release materials are generally relatively expensive and the types of solvent that can be used for diluting and coating fluorine-type release materials are typically limited to fluorine-type solvents due to solubility issues. Therefore, practical use of fluorine-type release materials is limited. Long chain alkyl group-type release materials typically consist of a copolymer of an acrylic ester, vinyl ether, and acrylamide derivative or the like and these are disclosed, for example, in Japanese Examined Patent Publication (Kokoku) Nos. 44-9599, 40-8903 and 43-21855. More specifically, Japanese Examined Patent Publication (Kokoku) No. 44-9599 discloses a release material consisting of a copolymer of octadecyl methacrylate and acrylonitrile. Japanese Examined Patent Publication (Kokoku) No. 40-8903 discloses a release material consisting of a copolymer of octadecyl vinyl ether and acrylonitrile. Also, Japanese Patent Publication (Kokoku) No. 43-21855 discloses a release material consisting of a copolymer of octadecyl acrylamide and acrylic acid.
However, these release materials are poor in anchoring to certain substrates, such as polyester. Furthermore, these release materials usually contain a group having high affinity for a pressure-sensitive adhesive, such as carboxyl group, nitrile group, amide group, or hydroxyl group. Such a group may drive the release material to readily transfer to the interface with, or into, an adjacent pressure-sensitive adhesive, or vice versa. As a result, the release material tends to be difficult to release from the pressure-sensitive adhesive or the adhesive properties, such as adhesive strength, of the pressure-sensitive adhesive tend to be decreased. These tendencies are generally more pronounced when the release material is heated to higher temperatures.
Japanese Unexamined Patent Publication (Kokai) No. 63-202685 discloses an acrylic release material obtained by crosslinking an acrylic copolymer of an acrylic ester and an acrylic compound having a functional group with a crosslinking agent. According to this publication, a crosslinking agent such as isocyanate is used for a crosslinking treatment. Crosslinking reactions involving isocyanates tend to proceed gradually and, thus, an undesirably high amount of time may be necessary for the desired performance, such as releasing strength, to be obtained in such materials. Furthermore, the above-described acrylic copolymer contains a polar group such as carboxyl group, hydroxyl group, or amide group, which may provide a higher release strength as compared with release strengths of silicone-type release materials. In addition, when an acrylic copolymer is crosslinked using an isocyanate, it tends to be difficult to release the release material from an acrylic pressure-sensitive adhesive. This tendency is generally more pronounced at a higher temperature.
Japanese Unexamined Patent Publication (Kokai) No. 11-152459 discloses a non-silicone type release material consisting of an acrylic copolymer that does not contain a polar group. This release material is obtained by polymerizing a release material forming composition containing (a) an organic compound having two or more (meth)acryloyl groups and having a number average molecular weight of 600 or more and (b) an alkyl (meth)acrylate under the irradiation of an active energy ray, such as ultraviolet ray or electron beam, while forming crosslinks through the (meth)acryloyl group, and curing the polymer. The release material of this publication contains no polar groups and even when exposed to high temperatures, the release strength is presumed to not significantly increase. However, organic compounds having two or more (meth)acryloyl groups do not effectively decrease the release strength. Although the monomer unit of alkyl (meth)acrylate, which is contained in the copolymer after curing, has a tendency for decreasing the release strength of the release material, the monomer of alkyl (meth)acrylate contained in the release material forming composition is not completely polymerized, even under irradiation of an active energy ray, and partially remains as an unreacted monomer. This may impair the stability of a residual adhesion strength of a pressure-sensitive adhesive after removal of the release material.
On the other hand, Japanese Unexamined Patent Publication (Kokai) Nos. 55-65281, 55-80479, 55-152775 and 55-155079 disclose a method for producing a pressure-sensitive adhesive tape or sheet where the release layer is a release material consisting of a polyolefin-based elastomer or a release material consisting of a mixture of a polyolefin-based elastomer and polyethylene and the adhesive layer is a pressure-sensitive adhesive mainly comprising a polyacrylic ester. The polyolefin-based elastomer used in these publications is one having a shear storage modulus according to a JIS K7213 test of 2.0xc3x97107 Pa or less and a surface wetting property such that the equilibrium contact angle to a standard solution having a surface tension of 50 mN/m for use in the JIS K6768 test is 55xc2x0 or more at a temperature of 20xc2x11xc2x0 C. and a relative humidity of 65xc2x15%. The release material disclosed in these publications is formed by extrusion molding and the release material obtained is not subjected to any treatment. The untreated release material has a problem in that the adhesion between this release material and a substrate supporting the release material is low. Furthermore, the release strength of the release material from the pressure-sensitive adhesive is relatively high and in particular, when exposed to a high temperature, the release strength is increased and the release material obtained disadvantageously fails in having a sufficiently low release strength.
The present invention provides a release material that is enhanced in its anchoring to a substrate, relatively low in its release strength from a pressure-sensitive adhesive (even after the exposure to a high temperature), and capable of allowing the pressure-sensitive adhesive to hold a stable residual adhesion strength. Also disclosed are release material articles using the same and a process for producing the article.
Specifically, the present invention provides a release material formed by irradiating a release material precursor having a shear storage modulus (hereinafter sometimes referred to as a xe2x80x9cstorage modulusxe2x80x9d or xe2x80x9cGxe2x80x2xe2x80x9d) of 1xc3x97102 to 3xc3x97106 Pa at 20xc2x0 C. and a frequency of 1 Hz, wherein the release material has a contact angle of 15xc2x0 or more when measured using a mixed solution of methanol and water (volume ratio: 90/10) having a wet tension of 25.4 mN/m.
The release material of the present invention is enhanced in its anchoring to a substrate by using treatment of radiation on an untreated release material precursor. Furthermore, by virtue of this treatment, the release material of the present invention exhibits relatively low release strength from a pressure-sensitive adhesive, even after the exposure to a high temperature. Still further, after the release of the release material from a pressure-sensitive adhesive, the pressure-sensitive adhesive can hold a stable residual adhesion strength.
The term xe2x80x9ccontact anglexe2x80x9d as used in the present application is defined as a contact angle measured using a mixed solution of methanol and water (volume ratio: 90/10) having a wet tension of 25.4 mN/m described in JIS K6768:1999. This measurement is conducted at a temperature of 23xc2x11xc2x0 C. and a relative humidity of 50xc2x15%.
In present application, the term xe2x80x9crelease strength from an acrylic pressure-sensitive adhesive,xe2x80x9d as used in the evaluation of the release property of the release material from a pressure-sensitive adhesive, means a value of release strength measured by the following method. A release material layer is applied to a pressure-sensitive adhesive sheet consisting of an acrylic pressure-sensitive adhesive (PSA). The PSA is produced by forming a copolymer from a monomer consisting of butyl acrylate, acrylic acid, vinyl acetate, and 2-hydroxyethyl acrylate at a weight ratio of 100:8:2:0.2 and adding isophorone diisocyanate corresponding to 1%, in terms of the mass ratio, to crosslink the copolymer. In this state, the sheet is left standing in an oven at 110xc2x0 C. for 6 hours. Thereafter, the sheet is left standing overnight at room temperature (25xc2x0 C.) and used as a sample for measurement. The release strength of this sample is measured at a release angle of 180xc2x0 and a release rate of 300 mm/min at room temperature (25xc2x0 C.). The value, thus obtained, is defined as the release strength.
Furthermore, the shear storage modulus (Gxe2x80x2) as used in the present application is a value measured using a viscoelasticity analyzer at 20xc2x0 C. and a frequency of 1 Hz.
In the present application, the terms xe2x80x9c(meth)acrylic acidxe2x80x9d and xe2x80x9calkyl (meth)acrylatexe2x80x9d are used to indicate an acrylic acid or a methacrylic acid, and an alkyl acrylate or an alkyl methacrylate, respectively. In addition, the term xe2x80x9cpolymerxe2x80x9d includes a binary copolymer, a ternary copolymer, a quaternary or greater copolymer and derivatives thereof. These polymers may also be used as a mixture.